Wireless transmission systems may use transmit diversity, whereby signals are simultaneously transmitted to a receiver using a plurality of transmit antennas. A transmitting modifying communication device may have multiple antenna elements that transmit signals to communicate information. Multiple antenna elements for transmission may enhance spectral efficiency and capacity, allowing for more users to be simultaneously served over a given frequency band, while reducing signal degradation caused by multi-path and fading. Transmit diversity parameters may be applied to signals transmitted from two or more antennas, and may modify an effective power distribution detected by receivers, such as base stations.
A receiving feedback communication device may receive and extract information from the transmitted signals, and provide a feedback signal to a transmitting device. Accordingly, some transmit diversity systems may use certain feedback signals from the receiver in order to adjust a transmit diversity parameter such as power ratio or phase difference. Knows methods may use, for example, the uplink power control, e.g., power control bits, or reverse power control signal, provided by a base station over the downlink to a mobile terminal as feedback for a transmit diversity parameter.
US Patent Publication No. 2003/0002594, entitled “Communication device with smart antenna using a quality-indication signal,” published Jan. 2, 2003 and assigned to the assignee of the present application, the contents of which are hereby incorporated herein by reference, describes using a power control signal, for example, as provided by the power control bit of the CDMA protocol, as a quality indication signal. In some situations, there may be drawbacks in using a power control signal as a signal quality indicator, and adjusting phase thereby.
According to the scheme described, if the power control bit (PCB) indicates “power down”, e.g., that the mobile should reduce power, the phase setting algorithm may interpret the phase difference associated with that PCB to be more desirable than if the PCB indicates a “power up”, e.g., that the mobile should increase power. The algorithm may assume the change in phase difference to be the dominant statistical reason the power was commanded up or down. As explained below, this may not always be the case, or the statistics may require long time to stabilize degrading transmit diversity performance. Therefore, although the use of the PCB for phase control may be effective, it may have some drawbacks that may be overcome by applying the present invention.
For example, the above and other references discuss using the power control feedback from the base station to the mobile station to determine the effect of the phase shift tests, e.g., perturbations. Some power control schemes typically use a one-bit feedback that causes a fixed, for example, 1 dB power change in the total transmission power of the mobile station. Therefore, for perturbations to be detected by the base station, phase shifts must be introduced that produce a perceived power change at the base station on the order of magnitude of 1 dB. For signals having equal power, this means approximately a phase shift going from perfect phase alignment to an offset of 60 degrees, thereby causing a 1 dB reduction in total power. This amount of phase shift may cause disruptions in the performance of the intended modulations. In addition, it may be desirable to identify the phase-induced power changes with the normal up/down power commands inherent in one-bit power control schemes. What is imbedded in the one-bit is the normal up/down power command, the response of the base station to overcome the effects of fading, the need of the base station to overcome ambient noise variations, and the result of the phase perturbations.
For example, if the base station previously issued a “power up” command (for example, based on signal fading), and the mobile station increases the power by 1 dB and simultaneously adjusts a value of a transmit diversity parameter that degrades reception at the receiver (relative to a previous value of the transmit diversity parameter), the base station may issue a “power down” signal. In such case, the mobile transmitter may mistakenly take this as a sign that the adjustment to the phase difference improved the signal quality.
Similarly, for example, if the base station previously issued a “power down” command (for example, based on signal fading), and the mobile station decreased the power by 1 dB and simultaneously adjusts a value of a transmit diversity parameter that improves reception at the receiver (relative to a previous value of the transmit diversity parameter), the base station may issue a “power up” signal. In such case, the mobile transmitter may mistakenly take this as a sign that the adjustment to the phase difference degraded the signal quality.
In these systems, therefore, the mobile station must perform complicated filtering and selection to deduce the resulting feedback information. There is therefore a need for improved mechanism for providing feedback to a transmit diversity communication device.